Advances in the Global Positioning System (GPS) have provided non-military users an inexpensive and portable location and tracking device. Currently the GPS system is used to provide directions to drivers through an in-vehicle system, provider location and tracking information for marine navigation, and allow shipping companies to locate and track individual shipments. However, the GPS system is severely limited in an indoor environment.
The GPS system relies on the timing of signals from GPS satellites received by individual GPS units on the ground. Thus, an unobstructed view to the satellites is necessary to receive the signal. In an indoor environment, such an unobstructed view is, in general, not possible to obtain. Furthermore, the principal objective in developing GPS was to offer the United States military accurate estimates of position, velocity and time. Civil users of GPS were to be provided only “reasonable” accuracy consistent with national security concerns. As a result, satellite signals are purposefully degraded under a government policy called Selective Availability and consequently the resolution provided by the system is no more than 100 meter for civilian users. This coarse resolution is inadequate for many applications and compounds the problem of the ineffectiveness of GPS indoors.
Because of these limitations, other technologies have been developed to locate and track users or objects in an in-building environment. One such system uses tags placed on the items that are to be tracked. The tags can be either active or passive. An active tag contains power circuitry, which can communicate with base stations. A passive tag contains no internal power, rather it is charged either inductively or electromagnetically as it passes within the range of a base station. Using this derived power, the passive tag communicates with the station. The base stations are physically linked together through a wired or wireless network. Each tag transmits a code uniquely identifying itself. Thus, the location of the tag is determined to be in the vicinity of the base station with which the tag last communicated.
Such tag-based tracking and location systems, while being useful in an in-building environment, require a significant installation of specialized base stations. A tag-based system can only determine the location of the tags as being “near” a particular base station, consequently, to achieve a sufficiently high resolution a large number of base stations must be installed. Obtrusive tags have to be placed on every item that is to be tracked or located, and in the case of infra-red tags, the system operates only when there is a “line-of-sight” between the tag and a base station. For all these reasons location-determination technology based on tags has had very limited success.
Therefore, the present invention is generally directed to a system for locating and tracking a user in a building without a specialized infrastructure and with the ability to track without a line-of-sight between the user and a base station.
The present invention is also generally directed to a system for locating and tracking a user in a building using the existing Radio-Frequency (RF) Wireless Local Area Network (WLAN) infrastructure.
A Wireless Local Area Network (WLAN) consists of base stations connected to a wired network, and mobile devices which are “connected” to the WLAN through wireless communication with the base stations. The present invention uses the signal sensing ability of both the base station and the mobile device to determine the location of the mobile device, and thus the location of the user of the mobile device. The strength of the received signal from several base stations is measured by the mobile device. The mobile device then compares the signal strength from each of the base stations to a pre-computed table containing the base stations' signal strength at various known locations of the mobile device. From this comparison, the mobile device determines its location. Alternatively, the signal strength from the mobile device can be measured at a number of base stations. This signal strength is then compared by a central computer to a pre-computed table containing the mobile computer's signal strength at the base stations for various known locations of the mobile computer. From this table, the central computer determines the location of the mobile computer.
Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments which proceeds with reference to the accompanying figures.